Cracking furnaces are used in particular in the production of ethylene. In the steam cracking process for ethylene, a hydrocarbon feedstock is diluted with steam, and then heated rapidly to a high temperature by passing it through tubes (usually referred to as “furnace coils”) in a furnace. The high temperature decomposes the hydrocarbon feedstock. The output stream, containing a broad mixture of hydrocarbons from the pyrolysis reactions in the pyrolysis tubes plus unreacted components of feedstock, is then quenched to prevent recombination of the products. The cooled stream can then be processed through a series of distillation and other separation operations in which the various products of the cracking operation are separated.
Known cracking furnaces suffer from a number of problems. Because of the very low residence time of the feedstock and steam flowing through the tubes in the furnace (a few tenths of a second), the furnace and the tubes must be maintained at a very high temperature in order to achieve the necessary rapid heating to achieve pyrolysis. A large amount of fuel is thus required to fire the furnace.
Further, the very high temperature of the tubes in the furnace leads to the deposition of coke on the inside of the tubes. This coking is particularly unwelcome, as the presence of a layer of coke on the inside of the tube reduces heat transfer from the furnace to the feedstock, and so affects yield. It also increases the pressure drop in the pyrolysis tube, although this factor is generally considered to be less significant than the effect on heat transfer.
If coke deposition is sufficiently severe, it is normally necessary to take a furnace out of service periodically (typically every 20 to 60 days) to allow decoking of the tubes (such as by steam cleaning). Since each furnace represents a very large capital investment, it is desirable to keep such downtime to a minimum.